1. Field of the Invention
The present invention relates generally to document processing systems, and more particularly to schemes for automatic color adjustment in document processing systems.
2. Related Art
A document processing system typically includes one or more image capture devices (ICDs) for obtaining digital representations (also called digital images, or simply images) of documents. The image capture devices include optical input devices, such as photocopiers and optical readers and scanners. The digital images are then processed by other components of the document processing system.
Conventionally, such processing includes an adjustment of the colors in the digital images. This color adjustment operation is needed to compensate for deficiencies in the original documents.
Consider, for example, a photograph. The color of white usually varies in a photograph. Also, typically, white areas in a photograph are not purely white, but include some tint. Consequently, what appears to be white in a photograph does not usually scan as pure white. Instead, the resulting digital image typically includes some tint. Removing this tint (i.e., correcting the "white point") operates to improve the appearance of the image. As is well known, "white point" refers to the color represented when the tristimulus values used to measure a color (such as red, green, and blue) are equal in value and are at their maximum values. Alternatively, if tristimulus values are measured according to the CIE metric L*, a*, and b*, then the white point is represented by L*=100, a*=0, and b*=0.
Similarly, the color of black in a photograph varies widely with exposure and development. Thus, what appears to be black in a photograph often does not scan as pure black. Correcting this (i.e., correcting the "black point") operates to improve the depth of the image. As is well known, "black point" refers to the color represented when the tristimulus values used to measure a color (such as red, green, and blue) are equal in value and are at their minimum values. Alternatively, if tristimulus values are measured according to the CIE metric L*, a*, and b*, then the black point is represented by L*=0, a*=0, and b*=0.
The color adjustment operation discussed above is also needed to compensate for deficiencies in the image capture devices. The white point of a scanner may not match the color white as represented in a given photograph. With some scanners, for example, the white point may be set such that most "white" things scan with a yellowish pink tint.
Also, some scanners have difficulty reporting the absence of light as a neutral black. As a result, with some scanners, the darkest part of a scan is often bluish-purple. Other scanners suffer from different color faults.
Thus, in conventional document processing systems, color adjustment processing is often performed to compensate for color-related deficiencies in the original documents, and/or in image capture devices.
Conventionally, such color adjustment processing is performed manually. Clearly, this is not an optimal solution since manual color adjustment processing is tedious and time-consuming.
Attempts have been made to automate the color adjustment process. Such attempts have been either incomplete or not entirely effective, or both.
For example, one such attempt involved identifying the brightest pixel in the digital image, and measuring the color of this pixel. If the color of this pixel was sufficiently close to a neutral white, then this pixel (and all other pixels of the same color) was scanned as pure white.
This approach is not entirely effective because the color adjustment is based on a single pixel. Consequently, measurement noise sometimes reduces the accuracy of the color adjustment. Also, this approach is not complete since it does not include black point adjustment.
The deficiencies of prior devices is represented, for example, in a commercial product called "Adobe Photoshop," an image editor used with the Apple Macintosh computer and with computers running the Microsoft Windows operating system. "Adobe Photoshop" provides an "Auto" option which they advertise as performing automatic color adjustment. However, this "Auto" option must be manually invoked by the user. Also, this "Auto" option does not produce satisfactory results. "Adobe Photoshop" also appears to provide a user-selectable option for adjusting the white and/or black points based on a single pixel. The flaws with this approach are discussed above.
Accordingly, what is required is an improved system and method for automated color adjustment in a document processing environment.